Lake Tahoe's iconic transparency and stunningly blue waters are often the first thing that comes to mind when people think of the Tahoe Basin. Natural disturbances such as fire, floods, and landslides allow fine sediments and nutrients to enter the lake. Urbanization and development have also altered the natural hydrologic regimes of many of Tahoe's watersheds. Studies completed as part of the Lake Tahoe Total Maximum Daily Load (TMDL) show that urban areas are the primary source of fine sediment, the pollutant known to impact lake clarity. A wide variety of programs aim to improve water quality in the Basin such as stormwater treatment projects and aquatic invasive species insection stations for boats entering the Tahoe Basin.